1. Field of the Invention
The present invention relates to a sheet separating device, a sheet feeding device, and an image forming apparatus.
2. Description of the Related Art
A sheet supplying device employed in an image forming apparatus such as a copier or a printer to supply a sheet to an image forming unit generally includes a sheet separating device. The sheet separating device separates an uppermost sheet from a bundle of sheets so that the sheet feeding device can supply the sheets one sheet by one sheet. To achieve high productivity and high reliability required of the sheet separating device, the sheet separating device generally adopts a frictional separation method that separates sheets using a sheet feeding roller or an air-assisted separation method that separates sheets by blowing air. The sheet separating device that adopts the air-assisted separation method typically blows separating air against a sheet sucked onto a conveying belt by a negative pressure, thereby separating the second and subsequent sheets.
As an example of such an air-jet-type sheet separating device, a sheet separating device including a shutter unit that is opened and closed at predetermined intervals to increase air-assist effect without increasing the size of an air blower is disclosed in Japanese Patent No. 4492429. More specifically, the sheet separating device includes a sheet feeding tray where sheets are to be placed in a stack, a feeding unit that feeds the sheets placed on the sheet feeding tray one by one from the top of the stack, a fan that blows air against an end surface of the stack of the sheets placed on the sheet feeding tray, and an air-blower control unit that temporarily increases an airflow rate of the air blowing unit. The air-blower control unit includes a shutter member, which closes and opens one of an air inlet port and an air outlet port of the fan, and a shutter driver. The shutter driver includes a solenoid and controls open/close switching of the shutter member by switching energization state of the solenoid. The air-blower control unit causes the shutter driver to close any one of the air inlet port and the air outlet port of the fan and, after a predetermined period of time, open the closed one of the air inlet port and the air outlet port, thereby temporarily increasing the airflow rate when the port is opened.
Meanwhile, conventional sheet separating devices can be poor in sheet separation capability and suffer from occurrence of what is referred to as “multi-feed”, i.e., two or more sheets are conveyed together, and therefore are incapable of maintaining stable conveyance performance. That is, the first sheet and the second sheet are conveyed together as a result of failing to separate the first sheet sucked onto a conveying belt from the second sheet lifted in the air.
Furthermore, although the technique disclosed in Japanese Patent No. 4492429 provides an advantage that sheet separation using air-assist effect is improved, the technique cannot solve the multi-feed problem the cause of which is poor separation capability.
Accordingly, there is a need for a sheet separating device that can cause less multi-feed and, therefore, improve separation capability and maintain stable sheet feeding performance.